Myelopoiesis replenishes peripheral supplies of myeloid phagocytes in the steady-state, as well as in response to increased demand during infection. Inflammatory mediators produced in response to microbial invasion act on hematopoietic stem and progenitor cells to stimulate myelopoiesis and mobilize myeloid phagocytes to combat infection. Recent data have demonstrated that hematopoietic stem cells, as well as more committed progenitors, express Toll-like receptors (TLRs), and that direct detection of microbial products by these cells can induce myelopoiesis. However, it is currently unclear whether the myeloid cells generated by direct microbial stimulation of progenitors are functionally equivalent to pre-existing myeloid cells. We have observed that hematopoietic progenitors transiently exposed to TLR2 or TLR4 agonists generate macrophages that make less TNF-1 and IL-6 upon TLR stimulation, but more ROS upon Dectin-1 ligation. Our data suggest that myeloid cell function can be altered for an extended period following microbial exposure. This may be an important mechanism for limiting the damaging effects of inflammation, but could also have a previously unappreciated impact on immune function, including the capacity to combat subsequent infections. In this project, we will define the mechanisms underlying this progenitor reprogramming. In Aim 1 we will define the mechanisms by which myeloid precursors are reprogrammed by transient exposure to TLR agonists. We will identify the precursor populations affected, determine whether reprogramming requires direct contact with TLR agonists, define the role of TLR adaptors and other microbial receptors, and determine how long progenitor reprogramming persists. In Aim2 we will determine the mechanisms underlying the altered responsiveness (enhanced oxidative burst but suppressed inflammatory cytokine production) of reprogrammed macrophages. These studies will pave the way for future investigation of the role of progenitor reprogramming in controlling inflammation during primary infection and in establishing susceptibility to subsequent infection, and for the therapeutic exploitation of myeloid progenitor reprogramming for the treatment of inflammatory disease. PUBLIC HEALTH RELEVANCE: Myeloid cells are important for fighting infection, but they also cause inflammation which can result in damage. We are defining how these cells are generated during infection, and how infection affects their function. Clinical manipulation of these cells may improve our ability to fight infection or prevent the damaging effects of inflammation.